1. Field of the Invention
This invention relates to a marking apparatus for printing markings on a substrate, and more particularly to an improvement in an electrostatic print marking apparatus.
2. Description of the Prior Art
It has been proposed by the inventors of the present invention to electrostatically print markings on rolled steel plates. Various kinds of information such as the name of the manufacturer, the destination of shipment, dimensions of the plate and so forth are printed in the form of markings on the successively manufactured steel plates fed out of a rolling mill. In a rolling mill, the rolled steel plates leave the rolling rolls in a heated condition. Therefore, in the process already proposed by the inventors, an intermediate image carrying medium is provided between the steel plate on which marking is to be finally printed and an electrostatic image recording member (drum or belt) so that a toner image formed on the recording member is first transferred to the intermediate image carrying medium and then to the steel plate. Thus, the electrostatic recording portion and the developing portion of the apparatus are protected from heat and mechanical vibration.
In the above described print marking apparatus, a high voltage is applied between the steel plate and the intermediate image carrying medium in order to effectively transfer the toner image from the intermediate image carrying medium to the steel plate. The strength of the electrostatic force effected between the image carrying medium and the steel plate to facilitate the transfer of the toner image is determined by the strength of the electric field and the strength of the charge carried by the toner particles. Therefore, the higher is the strength of the charge carried by the toner particles, the easier it becomes to transfer the toner image to the steel plate. Further, when the toner particles are sufficiently charged, the toner particles are prevented from scattering widely over the steel plate and accordingly the resolution of the image obtained on the steel plate is improved. The high voltage applied between the steel plate and the intermediate carrying medium in the above described print marking apparatus enhances the sharpness of the image obtained on the steel plate.
Nevertheless, as the toner image is transferred twice in the above described print marking apparatus, this apparatus suffers from a defect in that the charge of the toner particles is neutralized or weakened in the first transfer process, which results in lowering of the electrostatic force effected in the step of the second transfer of the toner image from the image carrying medium to the steel plate. The neutralization or the weakening of the electrostatic charge carried by the toner particles is caused by the discharge occurring in the step of the first transfer of the toner image from the electrostatic recording material to the intermediate image carrying medium. By the discharge, corona ions are generated and neutralize or weaken the charge carried by the toner particles.